But does Battle Ground take place on Halloween?
No, it's going to be immediately after PT... it was originally intended to be the second part of PT.
Can immortals only be taken out by mortals only on Halloween?
Only on a conjunction; Halloween is one, but not the only one. The Stone Table Battlefield is another, at least for Fae immortals (as Bob explains to Harry in CD - that's why Harry, Toot-toot, and the other pixies could kill Aurora in SK).
What happened before Halloween was invented?
Bob has mentioned that the "physics" of magic change over time. The conjunctions were likely different at one time.
Zeus took out Titans back in the day before All Hallows Eve was invented.
The Titans were arguably imprisoned rather than killed, though since Tartarus is also an "afterlife" location, it's not entirely clear.
(Although I think Tartarus as prison of the Titans is an earlier concept than Tartarus as a 'general' hellish afterlife.)
So can immortals take out other immortals any time they want?
I don't think so, in general. Bob in CD says that Mab and Titania created the Stone Table Battlefield specifically as a place immortals can die, so balances of power can change in Faerie.
Although CD does say that Titania could pull Mab into oblivion with her... I don't know. Maybe Mab and Titania coming into direct opposition is considered a conjunction? Or actually "automatically" summons the battlefield?
Because how these things work. When you are at your most vulnerable your enemies are at their most vulnerable too and you can achieve something.
Yeah. Bob says that on Halloween "the locked stasis of immortality" becomes malleable. That's when they can increase the power of their mantles etc. Normally they can't die but also can't grow.